Rotating machines often include rotors or shafts that are fabricated from steel forgings. However, large rotating machines, including large electrical machines, require a correspondingly large rotor and shaft, which would require a large steel forging. Such large steel forgings are expensive to forge and machine, heavy to transport, and offer limited flexibility in design of the machine. Furthermore, when solid forged steel rotors or rotor shafts are used with electrical machines, the bearings typically must be placed outside or beyond the end windings of the machine, which increases the length of the rotor and lowers its lateral critical speed.
Examples of rotating machine rotors are disclosed in U.S. Pat. Nos. 4,028,573 and 6,452,301; in International Publication Nos. WO2007/110282 and WO2011/012131; and in Japanese published patent application JP2006-158008A. Examples of composite shafts, armatures and tubes are disclosed in U.S. Pat. Nos. 3,623,220; 5,851,152; 6,072,252; 7,323,509 and 7,617,582; and in European Patent Application Publication No. 577409A1. Examples of magnetic bearings are disclosed in U.S. Pat. Nos. 3,787,100; 4,763,032 and 6,831,385. The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes.